Contrast-enhanced MRI has been proven to be a very sensitive modality for the early diagnosis of breast cancer in young women and/or in women with radiographically-dense breasts. The American Cancer Society recommends that high-risk women with lifetime risk greater than 20% should receive annual breast MRI screening. Depending on the practitioner and study characteristics, MRI may have a high false positive rate (with specificity reported as low as 81%), which can lead to many unnecessary procedures or even over-treatment. Positron emission mammography (PEM), the breast-specific high-resolution molecular imaging system originally invented by the PI of the submitted proposal, has been demonstrated to have a high negative predictive value. Integrating a PEM system into MRI for combined imaging is therefore likely to increase diagnostic accuracy. Furthermore, the recent commercialization of PET/CT and PET/MR clinical systems has led to wide acceptance in the oncology community for combined anatomic and molecular imaging. The goal of this application is therefore to design and construct a PEM system that could be used in conjunction with an MRI scanner. The developed PEM/MRI system in this project could be used in cancer detection, in personalizing care for breast cancer patients, and in the development of new drugs for prevention and therapy.